Re: GNAT "alignment value must be positive"

[Shark8 <onewingedshark@gmail.com>]

It's because "alignment 0" corresponds to an internal divide-by-zero error.
The LRM (13.4) gives the definition of the alignment portion of record-specification as:

      alignment_clause ::= at mod static_simple_expression;

So, "mod 0" is obviously erroneous in that case... however, that is the special case of records; in-general what we want is found in 13.3 which says, in part:

22/2  For a prefix X that denotes an object: 
23/2  X'Alignment
      The value of this attribute is of type universal_integer, and nonnegative; zero means that the object is not necessarily aligned on a storage element boundary. If X'Alignment is not zero, then X is aligned on a storage unit boundary and X'Address is an integral multiple of X'Alignment (that is, the Address modulo the Alignment is zero).

So, here the 0-alignment means that there is no imposed alignment WRT storage-element boundaries.

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All that said, it could be a bug where GNAT treats an aspect differently than the corresponding representation-clause/pragma.